Move `METADATA.toml` to `.data`

[srittau]

Currently, METADATA.toml is added to each top-level package. This was done, since we had no better place at the time. In https://discuss.python.org/t/extra-files-in-dist-info/39418/4 it turns out that it's probably possible to add a _typeshed directory into .dist-info, which would be a better location for it. But even better would be the .data directory.

hatchling supports this for wheels via the shared-data option. I'm not sure about setuptools or flit.

[srittau]

Although I'm not exactly sure what happens at installation time. From the Packaging User Guide:

The .data directory contains subdirectories with the scripts, headers, documentation and so forth from the distribution. During installation the contents of these subdirectories are moved onto their destination paths.